In the field of copper smelting, various methods have been proposed to recover copper from a copper-containing processing object (hereinafter, referred to as a “copper-bearing material) such as a copper ore or a copper concentrate. For example, when copper is recovered from a copper sulfide ore as one example of a copper-bearing material, the copper sulfide ore is usually processed through the following steps.
(1) Flotation Step
In the flotation step, a copper ore extracted from amine is ground and then mixed with water to prepare a slurry, and the slurry is subjected to flotation. The flotation is a separation process performed by adding a flotation agent containing a depressant, a frother, and a collector to the slurry and by blowing air into the slurry so that copper-containing minerals float and gangue sinks. As a result, a copper concentrate with a copper grade of about 30% can be obtained. The obtained copper concentrate is sent to the next fire refining step.
(2) Fire Refining Step
In the fire refining step, the copper concentrate obtained in the above flotation step is smelted in a smelter such as a flash smelter, and is then refined in a converter furnace and then in a refining furnace to obtain blister copper with a copper grade of about 99%. The blister copper is cast into anodes, and then the anodes are sent to the next electrolysis step. Arsenic contained in the copper concentrate is distributed to slag, dust, and blister copper by fire refining. The slag is granulated with water and used as a land-fill material or the like. The dust is returned to the furnace. Sulfur contained in the copper concentrate is separated as sulfur dioxide and used as a raw material for sulfuric acid.
(3) Electrolysis Step
In the electrolysis step, the anodes are placed in an electrolysis tank filled with a sulfuric acid acidic solution (electrolyte), and electrolytic refining is performed by applying electric current between the anodes and cathodes. By performing the electrolytic refining, copper contained in the anodes is dissolved and then deposited on the cathodes as electrolytic copper with a purity of 99.99% which is a product. At this time, arsenic that has been distributed to the anodes is eluted into the electrolyte. The eluted arsenic is recovered as decopperized slime by electrolytic copper removal. The decopperized slime is used as an intermediate material or returned to the furnace.
Arsenic distributed to slag in the fire refining step is fixed in a stable form. However, arsenic distributed to dust and decopperized slime is in an unstable form, and therefore it is undesirable to directly discharge the dust and the decopperized slime outside the system for disposal. For this reason, these dust and decopperized slime are returned to the furnace or separately processed. In this way, most of arsenic matter contained in the copper concentrate is finally distributed to slag and fixed in a stable form.
Meanwhile, raw material conditions have been changed in recent years. More specifically, the impurity content, especially arsenic grade, of copper ores tends to increase year by year, and the arsenic grade of copper concentrates obtained from copper ores is also becoming increasingly higher. For example, the arsenic grade of conventional copper concentrates is about 0.1 to 0.2%, but recently, it is not uncommon for copper concentrates to have an arsenic grade exceeding 1%. Therefore, even when the amount of a copper concentrate to be processed is the same as before, there is a case where existing slag treatment equipment for fixing arsenic to slag cannot cope with an increase in the arsenic content of the copper concentrate. Such a problem can be solved by, for example, providing new slag treatment equipment or increasing the capacity of the existing slag treatment equipment, but this requires a significant investment and therefore leads to an increase in cost.
It is considered that if the arsenic grade of a copper concentrate can be reduced to, for example, the same level as before by separating and removing arsenic in the process of obtaining a copper concentrate from a copper ore, the need for making such an investment can be eliminated and the existing slag treatment equipment can be operated without changing its initial arsenic processing load.
In this regard, Patent Document 1 discloses a method for separating arsenopyrite contained in iron pyrite by flotation. In this method, flotation is performed by adding a sulfuric acid-based depressant containing hydrogen sulfite ions, such as sodium hydrogen sulfite, to iron pyrite under conditions where the pH of a slurry is maintained at 8 or less and the temperature of the slurry is set to 30° C. or higher so that arsenopyrite is separated from the iron pyrite.
However, it is difficult to directly apply this method to separation of arsenic from a copper ore or a copper concentrate. This is because, in the case of, for example, a copper concentrate mainly containing chalcopyrite or bornite, arsenic is often present as an arsenic mineral such as tennantite ((CuFe)12As4S13) or enargite (Cu3AsS4), and these arsenic minerals have floating properties similar to those of chalcopyrite or bornite, and therefore it is difficult to separate arsenic and copper from each other by flotation.
Patent Document 2 discloses a method in which an arsenic-containing copper concentrate is heated at 90 to 120° C., and then potassium hexacyanoferrate (II) (yellow prussiate of potash: K4[Fe(CN)6]) is added as a depressant for suppressing the flotation of copper in an amount of 10 to 15 kg per ton of the copper concentrate to float an arsenic mineral to separate it from chalcopyrite or bornite that sinks.
This method uses oxidization of a surface of the copper mineral in a copper concentrate by heating, which forms an inactive oxide film on the surface. It is considered that this inactive oxide film causes the difference in surface chemical state or crystal chemical state between the surface of the copper mineral and a surface of an arsenic mineral, which causes difference in floating properties in subsequent flotation process. However, when practically used, this method requires equipment and energy for heating a large amount of copper concentrate, which causes a problem such as an increase in cost.
Patent Document 3 discloses a method for suppressing the flotation of an arsenic mineral in which a non-ferrous metal sulfide mineral containing arsenic is subjected to flotation at a pH of 9 to 10 by adding air, hydrogen peroxide, another oxidizer, xanthate as a collector, and a mixture of a polyamine and a sulfur compound as a depressant. This method mainly describes a method for separation between a nickel sulfide mineral and an arsenic mineral, but does not describe separation between a copper mineral and an arsenic mineral.
Non-Patent Document 1 discloses a method for performing flotation in which a copper mineral-containing slurry is treated with hydrogen peroxide, and then the pH of the slurry is adjusted to 5 by adding sodium nitrate. This non-Patent Document also proposes a method for performing flotation in which hydrogen peroxide and EDTA are added to a copper mineral and then pH is adjusted to 11 with potassium hydroxide. However, these two methods have problems in cost and safety during handling of deleterious substances.
As described above, it is difficult for any of the above methods to efficiently separate an arsenic mineral from a copper-bearing material by flotation.